$AzureDevOpsPAT = "***" # Replace the values in this for the variables with your own.
$OrganizationName = "" # Replace the values in this for the variables with your organisation name.
$ProjectName = "" # Replace the values in this for the variables with your project name.
$AzureDevOpsAuthenicationHeader = @{Authorization = 'Basic ' + [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes(":$($AzureDevOpsPAT)")) }
$Uri = "https://dev.azure.com/$($OrganizationName)/$($ProjectName)/_apis/build/builds?statusFilter=Started&api-version=5.1"
$PendingJobs = Invoke-RestMethod -Uri $Uri -Headers $AzureDevOpsAuthenicationHeader -Method get -ContentType "application/json"
$JobsToCancel = $PendingJobs.value
ForEach ($build in $JobsToCancel) {
    $build.status = "Cancelling"
    $body = $build | ConvertTo-Json -Depth 10
    $urlToCancel = "https://dev.azure.com/$($OrganizationName)/$($ProjectName)/_apis/build/builds/$($build.id)?api-version=5.1"
    Invoke-RestMethod -Uri $urlToCancel -Method Patch -ContentType application/json -Body $body -Header $AzureDevOpsAuthenicationHeader
}